The analysis, available in the report "Growth and Dispersion of Accountable Care Organizations: August 2013 Update," shows there are 488 total ACOs across the country. Of those, 253 have contracts with CMS, meaning Medicare ACOs account for 52 percent of total ACOs.

"Since the inception of the accountable care concept, non-Medicare ACOs…have been dominant," the report states. But with CMS announcing hundreds of new Medicare Shared Savings Program participants, Medicare ACOs surged ahead this year.

However, the report notes commercial ACOs continue to develop and are announced on a rolling basis, while there will be no new Medicare ACOs until January 2014. "This may shortly make non-Medicare ACOs a majority once again," the report states.